Alkaline batteries of a desired voltage may be formed from a plurality of series-connected cells. One technique which has been used for forming such batteries is to vacuum seal a positive electrode, a dielectric, and a negative electrode in a plastic package to form a single battery cell. The negative electrode is typically formed of a pasty mixture of amalgamated zinc powder, potassium hydroxide solution, and a jelling agent sealed in a plastic bag. One method for interconnecting such cells to form a battery is to provide a pin or other pointed conductor which is electrically connected to the positive electrode of the cell and which projects through the vacuum-sealed plastic package on the positive electrode side of the cell. When the cells are stacked to form a battery, they are stacked with the negative electrode side of one cell adjacent to the positive electrode side of the next cell. As the cells are pressed together, the pointed conductor of one cell pierces the package and negative electrode bag of the adjacent cell to effect a series electrical connection between the cells. A battery of this type is described in greater detail in UK Pat. No. 2,097,574 and various modifications to this basic construction are discussed in U.S. Pat. Nos. 4,505,996; 4,525,439; and 4,554,226.
While batteries constructed in accordance with the teachings of the patents indicated above may provide satisfactory results, there is a basic drawback in the design of these batteries which has caused problems in industrial production. These problems result from the fact that the plastic foil covering the cell, which may for example be a laminate of polyamide and polyethylene, is a pliable and tough material. This material resists piercing of the contact pin or other pointed conductor. Some resistance to piercing is also provided by the bag enclosing the negative electrode. This results in the plastic foil covering, the bag, and the adhesive or other sticky material utilized for sealing the opening, being dented and deformed by the pin before piercing or breakthrough occurs. As a result, a thin sheath-like layer is formed about the contact pin by the adhesive and the various pierced material layers when the pin pierces into the negative electrode paste. This layer may provide an insulating cover over a substantial part of the surface of the contact pin which may completely prevent electrical contact from being made or may substantially limit the area of this contact and thus adversely effect the capacity of the battery.
Further, the denting of the cover layers before piercing occurs causes the soft, pasty mix of the negative electrode to yield, a pit thus being formed in the negative electrode mass as it is radially pushed outward from the penetration point. The various layers being pierced sag correspondingly, resulting in a pattern of radial grooves being formed in these layers at the moment of breakthrough. When breakthrough finally occurs, these grooves may remain as air channels directed from the contact pin outwards. Such channels may even reach the air outside the battery if the insulation between the cells is uneven or has any discontinuity or if the cells are not absolutely parallel to each other, or the pin bends slightly so that the pin enters the pierced cell at a slight angle rather than perpendicularly. The denting of the pierced layer prior to breakthrough may also cause the hole formed in these layers to be somewhat jagged rather than cleanly-cut around the pin, making it far more difficult for the resulting opening to be sealed by the adhesive or other sealing layer provided between the cells.
Since the electrolyte of alkaline batteries, such as alkaline-manganese batteries, has a tendency to creep on metallic surfaces (the Marangoni Effect), over time electrolyte from the pierced cell will tend to ooze along the metal contact pin to the radial grooves. In the assembled battery, the cells are subject to a pressure forcing them tightly together, which may gradually cause the alkaline electrolyte to creep along the grooves and between the plastic covers of the two cells out of the battery. The loss of electrolyte adversely effects the capacity of the battery, and reduces both its shelf life and active life. It has been found that the process described above begins to take place within a few hours at temperatures of +71.degree. C., the temperature used in international quality tests.
In addition to permitting the escape of electrolyte, the channels may also permit carbon dioxide, which is always present in the surrounding atmosphere, to enter the cell, thereby reacting with the electrolyte and producing alkali metal carbonate crystals which expand the leakage paths of the electrolyte from the cell. This causes deterioration in deliverable energy from the battery, and is another potential problem with existing designs.
Finally, to the extent there is difficulty in properly sealing the enlarged hole formed as a result of the denting of the pierced layers, electrolyte may flow along the contact pin into the adjacent cell, partially shorting the two cells and further reducing the output capacity of the battery.
A need therefore exists for an improved construction for alkaline batteries and the cells thereof which prevents the denting of the cover layers prior to penetration, and thus (i) permits a contact pin to enter the negative electrode paste without a covering insulating layer, thus making good electrical contact therewith; (ii) prevents pitting of the negative electrode paste so that it makes good physical and electrical contact with the pin; (iii) provides a clean, easily sealable hole in the pierced layers; and (iv) prevents formation of grooves in the pierced layers through which electrolyte may seep out and carbon dioxide containing air may enter the cell.